A duplexer is generally known to be used to appropriately select a desired frequency band of transmitted and received signals in a communication radio. By way of example, and with reference to FIG. 1, a typical architecture for a code division multiple access (CDMA) cellular transceiver for operation in a cellular frequency band is illustrated. A modulated baseband signal is filtered, up-converted and amplified to a desired signal intensity before being fed to the duplexer. The duplexer selects a signal within a desired signal band and then transmits the signal through an antenna as a modulated carrier signal. Conversely, a received signal, appropriately selected by the duplexer, is amplified, filtered and eventually demodulated at the baseband.
A SAW duplexer may perform the selection of the desired signal band through a frequency filtering process comprising two SAW filters operating at different frequency bands. By way of example, the duplexer may have a received filter covering a passband from 869 MHz to 894 MHz and a transmitted filter covering a passband from 824 MHz to 849 MHz for a Cellular CDMA radio. For the personal communication services (PCS) CDMA radio, the received filter is generally set to cover the band from 1930 MHz to 1990 MHz while the transmitted filter covers the band from 1850 MHz to 1910 MHz. Thus, the SAW duplexer typically enables the simultaneous receipt and transmittance of the communication signals. In addition to providing the filtering selection of the appropriate signal band, the duplexer must also provide a desirable isolation between the received and transmitted channels. The received filter allows the receipt of the incoming signal and at the same time it must block any interference from the transmitted signal. Similarly, the transmitted filter allows the signal to be transmitted and simultaneously must block the interference from the received signal. Isolation is a measure of a desirable performance requirement in a duplexer. This is particularly the case for the CDMA mobile phone transceiver. In this type of transceiver, the incoming signal may be very weak, while the transmitted signal is generally very strong typically, 26–30 dB. Any leakage of the transmitted signal to a received channel could easily over-load the low noise amplifier. Therefore, a phase matching network is commonly used in SAW duplexers to provide isolation between the transmitted and received bands. While the transmitted and received filters of the duplexer are designed to be around 50 ohms, the phase matching network provides an impedance transformation at the outer band of the filter, and without a significant impact on the degradation of the passband characteristics. For example, while the impedance of the received filter is matched to be approximately 50 ohms across the passband, the impedance of the received filter should be very large at the transmitted frequency band. A microstrip line is generally embedded into the SAW duplexer package to perform the impedance transformation through phase shifting. To transform such a large swing of impedance, it is not uncommon to find devices with a strip length in the order of a quarter wavelength. The microstrip is generally embedded along the boundary edge of the package so as not to interfere with the SAW duplexer performance.
By way of example, U.S. Pat. No. 5,859,473 to Ikata et al. discloses two SAW filter die assembled in separated chambers of a multi-layered ceramic package. The separated filter die has the advantage that it minimizes any coupling between the transmitted and received filters. However, as the demand for miniaturization of SAW duplexers is enhanced, there is a need to have the two SAW filters incorporated into a single monolithic chip. Maintaining sufficient isolation between the two SAW filters is then complicated by a single chip implementation of the SAW duplexer. U.S. Pat. No. 6,466,103 to Iwamoto et al. discloses a SAW duplexer with a monolithic chip with a dimension of about 5 mm (width)×5 mm (length)×1.5 mm (height) and an arrangement of the phase matching line pattern at a position that is connected to bonding pads that are situated at a maximum distance from each other to reduce the interference between the two filters coupled through the matching line pattern.
To meet the demand for further size reduction of the SAW duplexer to a dimension of 3.8 mm (width)×3.8 mm (length) and 1.5 mm (height), as the SAW filters are laid closer together, the problems associated with maintaining an adequate isolation become even more complex. With less room to implement a phase matching line pattern that has to be approximately a quarter wavelength in length, it is more difficult to minimize the coupling between the filters through the line pattern. Furthermore, with the further shrinkage in the die size, second order effects have been discovered which degrade the isolation in both the transmitted and received bands of the duplexer.
By way of further example and as illustrated with reference to FIG. 2, a CDMA Cellular SAW duplexer chip mounted in a multi-layered ceramic package is presented. The SAW duplexer includes a received filter which covers the frequency band of 869 MHz to 894 MHz and a transmitted filter covering the passband of 824 MHz to 849 MHz. A phase matching network is embedded between the multi-layered package. FIGS. 3 and 4 illustrate frequency responses and isolation performance of the SAW duplexer of FIG. 2. As can be seen, the isolation performance immediately beneath the receiver band is only slightly over 45 dB. The degraded Rx isolation results from capacitive and inductive coupling between the transmitted filter and the received filter, as well as coupling within the transmitted filter and received filter.